How high should the SCN population density (egg count) be before growing a SCN-resistant soybean variety?

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Here at ISU, we have two management recommendation for SCN-infested fields. The recommendation given is based on the SCN egg population density (egg count) of the sample submitted.

Recommendation #1 - for fields with egg densities up to 5,000 eggs per 100 cc soil

We recommend that growers follow a six-year crop rotation scheme as illustrated below:

  • 1st year - Nonhost crop (such as corn)
    2nd year - 'PI88788' Resistant soybean
    3rd year - Nonhost crop
    4th year - 'Peking' Resistant soybean
    5th year - Nonhost crop
    6th year - Susceptible soybean

Recommendation #2 - for fields with egg densities greater than 5,000 eggs per 100 cc soil

  • We advise that farmers grow several years of a nonhost crop, such as corn, and to sample after harvest in the fall. When the SCN egg population densities drop below 5,000 per 100 cc soil, we then feel comfortable recommending that the grower follow the six year rotation listed above.

Many folks question why we recommend the six-year crop rotation scheme involving SCN-resistant varieties (recommendation #1 above) at very low SCN egg population densities. In my opinion, there are several reasons why such a practice makes sense. First, results of field research conducted by Dr. Terry Niblack (University of Missouri) when she was at Iowa State University showed that there can be soybean yield reductions at SCN egg population densities of less than 150 eggs per 100 cc soil. Consequently, it is possible that Iowa growers also can experience yield losses in fields with low SCN population densities. Secondly, SCN population densities can increase from a few hundred eggs per 100 cc soil to several thousand eggs per 100 cc soil in a single growing season. Growing a SCN-susceptible variety in a field that is known to be infested with SCN likely will allow the infestation to increase 100-fold or more, making profitable soybean production on that field much more difficult in future years. Finally, there is no real "yield penalty" for growing many resistant soybean varieties in fields with low SCN population densities. Results of SCN-resistant soybean variety trials conducted on both SCN-infested and noninfested fields throughout Iowa indicate that many SCN-resistant varieties yield close to, or as well as, SCN-susceptible varieties in noninfested fields and fields with low SCN infestations. Consequently, a grower is not really going to suffer reduced soybean yields when growing SCN-resistant soybean varieties in fields with low SCN egg population densities.

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